In working machines such as a mobile crane, a boom of a lattice structure is raised and lowered. A lattice boom has a rectangular sectional shape, and a main pipe is arranged at each of four corners of the rectangle. The main pipes are joined to each other by a lattice pipe.
To improve the buckling strength of a lattice boom, it suffices to increase the diameter of a main pipe or lattice pipe or to increase the plate thickness. However, in such cases, the weight of the lattice boom increases. Therefore, the weight of a suspended load that can be lifted with the same tension decreases, thus decreasing the crane performance.
Japanese Unexamined Patent Publication No. 2011-11911 discloses a mobile crane in which a plate member is connected to the inner circumferential surface of a main pipe. Accordingly, deformation of the main pipe due to buckling that occurs along the connecting direction of the plate member from the outer side toward the center of the main pipe can be suppressed.
Japanese Unexamined Patent Publication No. H3-13676 discloses a reinforcing structure for a truss structure. In the reinforcing structure, a reinforcing rib is formed at the outer circumferential surface of a main truss member. Accordingly, part of load to be borne by the main truss member is borne by the reinforcing rib, and therefore the truss structure can be configured of the main truss member with a small diameter.